Semiconductor wafers are substantially circular slices of monocrystalline semiconductor material formed by slicing transversely the single crystal ingots which are the product of the crystal fabrication process. These wafers are the basic medium from which large numbers, hundreds or thousands, of individual semiconductor devices are made using well-known dielectric coating, photolithographic masking and impurity introduction techniques. These semiconductor wafers are highly polished on both faces and when prepared for introduction into the device fabrication process, in current practice, range in thickness typically from about 30 to as thin as 10 mils and are from 2 to 4 inches in diameter.
From this point in the fabrication process to the point at which the wafer is divided into a large number of individual device chips, these wafers, which have acquired considerable value, are transferred through a variety of processing stations. They are subjected to various steps including selective coating, metallization, impurity introduction, selective removal of surface materials and cleaning operations. The wafers are relatively fragile and their surfaces are readily damaged by mishandling with the result that they may be unusable for device fabrication.
The wafers are stored and processed in part in trough-like plastic containers, termed cassettes, which have transversely-disposed grooves to retain each individual wafer in spaced-apart relation. Many operations however, require placing wafers flat on a platform or plate such as is used in a sputtering or evaporation station. Heretofore, it has been the practice to utilize vacuum wands or special tweezers to handle individual wafers. This is a slow and tedious process, dependent to a considerable extent upon the performance of a skilled operator and open to the possibility of damage to the wafers.
Accordingly, an object of this invention is the transfer of semiconductor wafers from a cassette type of holder to the processing platform to which the wafer is delivered in a horizontal disposition at a specific location without the manual kind of wafer-by-wafer handling which has been the previous practice.
It is also an object to pick up individual wafers and arrange them in a spaced-apart stack for ready transfer to a cassette.